Beneficial Effects of Exogenous Melatonin on Overcoming Salt Stress in Rice Seedlings (Oryza sativa)

Salinity negatively impacts the growth and development of rice during early growth stages that can be lessened through melatonin (MT), a hormone found naturally in plant and animal cells, which acts as a potential exogenous biostimulant on plants since it aids to fight against various abiotic stresses. The current research explored the potential impact of exogenouly applied MT on promoting early-stage rice growth under salinity conditions. At seedling stage, rice seedlings were exposed to 100 mM NaCl or maintained under normal conditions for 15 days and also pretreated with or without 100 µM MT. The findings of this study revealed that NaCl-stress hampered the growth of rice seedlings including shoot and root growth, reduced photosynthetic capacity. Additionally, exposure to salinity induced osmotic stress in rice, as indicated by increased proline accumulation, and oxidative stress, as evidenced by elevated hydrogen peroxide (H2O2) levels, increased lipid peroxidation, and reduced antioxidant enzymatic activities. However, application of MT markedly enhanced the overall growth performance and biomass under salinity condition. Plants treated with MT and salt stress showed augmented photosynthetic pigment content, decreased proline content and reduced H2O2 content contributing to reduction of oxidative damage. Moreover, MT application reduced the membrane damage under salinity conditions as reflected by remarkably reduced amounts of malondialdehyde in salt-exposed rice seedlings. The protective effect of MT against oxidative stress was associated with increased activities of antioxidant enzymes, including catalase, ascorbate peroxidase, and peroxidase. Our study reveals that the application of exogenous MT develops the resilience of rice seedlings to salinity stress by improving antioxidant defense systems which suggests that applying MT could be a dynamic tool for rice cultivation in saline-affected areas.